Electrical resistor and method for its manufacture

ABSTRACT

SMD current-sense resistors are manufactured by dividing a drawn laminated wire structure that consists of a core of a resistive alloy and a jacket made of copper, e.g. After removing part of the copper jacket, jacket sections used as connector contact layers remain only at the ends of the resistor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention pertains to an electrical resistor according to thepreamble of claim 1, as well as a method for manufacturing resistors ofthis type. Specifically, the invention pertains to low-resistancecomponents or shunts that are used in measuring current and are alsosuitable for surface mounting (SMD technology).

[0003] 2. Background Art

[0004] Precision-measurement resistors of this type which, if sorequired, also must withstand high loads and typically have a resistancevalue in the milliohm range, for example, between 0.5 mO and 10 m (or incertain cases, even higher or lower), consist of one of the conventionaland proven alloys used for this purpose, e.g., CuMnNi. The connectorcontact elements for the surface mounting of the resistor consist of ametal with high conductivity, particularly, copper. Until now,significant cost was required to manufacture known components of thistype (see, for example, EP 0 654 799 or EP 0 841 668).

SUMMARY OF THE INVENTION

[0005] The invention is based on the objective of allowing themanufacture of large quantities of such resistors at the lowest possiblecost.

[0006] This objective is realized with the characteristics disclosed inthe claims.

[0007] The connector contact elements of the resistor consequentlyconsist of jacket sections that are formed of a welded band or tubematerial and encompass the usually cylindrical resistor element intubular fashion preferably over its entire circumference. The resistorpreferably consists of a drawn laminated wire structure, the jacket ofwhich is removed between the connector contact elements such that theremaining jacket sections only consist of the connector contact elementsthat are separated from one another.

[0008] The described resistors not only can be manufactured very easily,e.g., of a continuously supplied laminated wire structure, but also havevery high mechanical stability and can be subjected to high electricloads. In addition, the desired resistance value can very easily beadjusted over a broad range during the manufacture of the resistors, forexample, by turning the resistor to a corresponding diameter.

[0009] The manufacture of the laminated wire structure utilized inaccordance with the invention preferably takes place in the manner knownper se for the manufacture of welding electrodes 9DE 197 12 817 C2, DE198 10 342 A1). In this case, a bright-cleaned metal band of copper orcopper alloy is continuously shaped into a tube and welded by means ofconventional welding methods, e.g., TIG welding, plasma welding,induction welding or laser welding. Before the welding process, ametallically bright core that may consist of a compact wire or alaminated wire structure of several metals or alloys is inserted intothe still unfinished tube. The copper jacket may be formed either bywrapping a band around the core in spiral fashion or by forming a bandinto a tube with a longitudinal seam. The jacket and the core aremechanically bonded into a laminated wire structure in a subsequentcommon shaping process, e.g. drawing or rolling. The manufacturedcompound wire thus formed is then conventionally drawn to the requiredfinal dimensions.

[0010] A laminated structure with relatively high mechanical stabilityis formed no later than during the drawing process. In order to producean even more solid connection, a diffusion layer may be formed betweenthe resistive alloy and the connector contact metal by annealing thelaminated wire structure.

[0011] The invention is described in greater detail below with referenceto the schematic embodiments that are shown in the figures on a greatlyenlarged scale.

BRIEF DESCRIPTION OF TEH DRAWINGS

[0012] The drawing figures show:

[0013]FIG. 1, a longitudinal section through a resistor according to afirst embodiment example,

[0014]FIG. 2, a side view of the resistor according to FIG. 1;

[0015]FIG. 3, a practical embodiment of a plastic cap for the resistoraccording to FIG. 1;

[0016]FIG. 4, a second embodiment example, and

[0017]FIG. 5, a section through FIG. 4 along plane A-A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIEMNTS

[0018] The resistor shown in FIG. 1 contains a longitudinally extendedresistive element 1 that has the form of a stepped rod as shown in thefigure and consists of a known resistive alloy, for example, CuNi, inparticular, CuMnNi, wherein a cylindrical central section 2 is locatedbetween two cylindrical end sections 3 of larger diameter. The lengthand diameter of the central section are dimensioned in accordance withthe desired resistance value. The two end sections 3 are each surroundedby a tubular jacket section 4 of identical axial length that consists ofa material with superior electrical conductivity, e.g., copper. Thejacket sections 4 are bonded onto the end sections 3 in mechanicallyrigid fashion and may also be alloyed to the resistive element, asdescribed further below, in order to achieve an even better connection.

[0019] The resistive element 1 is surrounded by an insulating body 6 ofheat-resistant plastic e.g., between the jacket sections 4, wherein theinsulating body has at least one flat outer surface for surface-mountingthe resistor, i.e., for attaching the resistor to a circuit board in theform of an SMD component. In FIG. 2, the insulating body 6 isschematically illustrated in the form of a cap with a square perimeter.The flat outer surface 7 of the insulating body 6 simplifies placementof the component on the circuit board at the intended location duringthe surface mounting process, since the insulating body prevents theresistor from rolling away. It is practical for the outer surface 7 tobe arranged flush with the circumference of the jacket sections 4, asshown in the figure. However, other arrangements are also conceivable.

[0020] One practical embodiment of a one-piece cap 6′ that can be usedas an insulating body 6 is shown in FIG. 3. This cap contains acylindrical recess 8 that corresponds to the shape of the centralsection 2 in FIG. 1, wherein a longitudinal slot 9 extends into thiscylindrical recess from another surface 7′. The width of thislongitudinal slot is such that the cap 6′ can be pressed onto thecentral section 2 while it is spread apart.

[0021] Resistors of this type with resistance values in the milliohmrange or below may, have a length of approximately 10 mm, e.g. whereinthe central section 2 located between the copper jacket sections 4 thathave a length of approximately 2 mm, e.g., may have different diametersof a few millimeters depending on the desired resistance value. However,the lengths and diameters may also be significantly larger or smaller.

[0022] In order to manufacture the resistor shown, a laminated wirestructure is initially manufactured, for example, in accordance with themethod described in DE 197 12 817 C2. This laminated wire structureconsists of a wire core of the alloy that subsequently forms theresistive element 1 and of a welded copper jacket that subsequentlyforms the jacket sections 4. The laminated wire structure is drawn tothe desired diameter in a drawing machine.

[0023] The drawn laminated wire structure is then annealed at atemperature that is at least as high as the recrystallizationtemperature of the resistive alloy (e.g., on the order of 600EC for acertain suitable CuMnNi alloy). This causes a diffusion layer to form inthe boundary layer between the resistive alloy and the copper jacket.Thus, due to this diffusive annealing, both metals can be combined likean alloy.

[0024] The laminated wire structure is then again drawn to a smallerdiameter in a drawing machine before it is processed further. Thisdrawing process may, among of the things, serve for hardening the wirebecause a wire consisting of the metals cited above by way of examplemay not have sufficient hardness for being turned to size.

[0025] In order to produce the resistors, the laminated wire structurethat corresponds to the cross section through the end sections 3 withthe jacket sections 4 is separated into individual parts with the lengthof the resistor according to FIG. 1. The axially central section of thejacket between the sections 4 must be removed before, during, or afterthis separation such that the remaining jacket sections can be used aselectrical connector contact layers of the resistor.

[0026] One practical means of removing the central section of the jacketand of dividing the wire into individual resistors consists of turningthe central section of the jacket to size and then cutting off theindividual resistors in a suitable automated lathe. The laminated wirestructure is fed to this lathe in continuous cycles. The central section2 of the resistive element 1 is usually turned to a diameter thatcorresponds to the desired resistance value and is smaller than thediameter of the laminated wire structure core and consequently that ofthe end sections 3 as shown in FIG. 1.

[0027] The insulating body 6 is ultimately attached in order to completethe resistor that can be used as an SMD component. For example, theplastic cap 6′ is clamped onto the central section 2 in the direction ofthe two arrows shown in FIG. 3, i.e., from a position in which it isparallel to the axis of the resistive element 1. This may also berealized in an automated process.

[0028] The finished resistor is annealed (age-hardened) again before orafter the insulating body 6 is attached in order to electricallystabilize the resistor in a manner is considered conventional forresistance wire.

[0029] Another means of manufacturing resistors of the described typefrom a laminated wire structure is described below with reference toFIGS. 4 and 5. In this case, the laminated wire structure that ismanufactured in the previously described manner and preferably annealedis not divided into the longitudinal sections shown in FIG. 1, butrather into individual flat disks. These disks consist of the centralresistive element 10 and the two jacket sections 14 according to FIG. 4,e.g., of copper. In order to produce the resistor, the sections 14′ ofthe annular copper jacket that were originally located between thejacket sections 14, i.e., opposite one another on the diskcircumference, are removed together with the adjacent partial regions10′ of the resistor core.

[0030] For example, it is possible initially to divide the laminatedwire structure into disks of circular shape as indicated by the brokenlines in FIG. 4, wherein the regions 10′ and 14′ are then conventionallyremoved in a suitable manner (e.g., by means of punching). However, itwould also be conceivable to carry out this process in the reversesequence.

[0031] The resulting resistors with the flat shape shown in FIG. 5 arevery suitable for use as SMD components, particularly, without theplastic cap used in the embodiment according to FIG. 1.

What is claimed is:
 1. Electrical resistor, in particular, for measuringcurrent, with a resistive element (1) that consists of a metal resistivealloy, and with two connector contact elements that are arranged at adistance from one another on opposite ends (3) of the resistive elementand consisting of a metal with a higher electrical conductivity than theresistive alloy, characterized by the fact that jacket sections (4)which surround the circumference of the resistive element (1) in tubularfashion and are formed of welded band or tube material are provided asthe connector contact elements.
 2. Resistor according to claim 1,characterized by the fact that the resistor consists of a drawnlaminated wire structure, the jacket of which is removed between theconnector contact layers (4).
 3. Resistor according to claim 1 or 2,characterized by the fact that the resistive element (1) is surroundedby an insulating body (6) between the jacket sections (4), wherein saidinsulating body has at least one flat outer surface 97) for placing theresistor onto a circuit board in the form of a surface-mount device. 4.Method for manufacturing electrical resistors by utilizing a wirematerial of a metal resistive alloy, characterized in that a wire coreconsisting of the resistive alloy is surrounded by a jacket of a metalthat has a higher electrical conductivity than the resistive alloy; inthat the jacket is welded such that a closed tube is formed; in that thelaminated wire structure formed of the wire core and its jacket is drawnto a smaller diameter, and in that the drawn laminated wire structure isseparated into parts that form the individual resistors; wherein anaxially central section of the jacket is removed on each individualresistor such that jacket sections (4) that are separated from oneanother and serve as connector contact elements remain only at the ends(3) of the resistor.
 5. Method according to claim 4, characterized bythe fact that the central jacket section is removed by means of turning.6. Method according to claim 4 or 5, characterized by the fact that,during the removal of the central jacket section, part of the underlyingresistive material is also removed until the desired diameter isobtained.
 7. Method according to one of claims 4-6, characterized by thefact that a body (6) of insulating material which has at least one flatouter surface 97) is applied onto the section (2) of the resistiveelement 91) which is located between the jacket sections 94).
 8. Methodfor manufacturing electrical resistors by utilizing a wire material of ametallic resistive alloy, characterized in that a wire core consistingof the resistive alloy is surrounded by a jacket of a metal that hashigher electrical conductivity than the resistive alloy 6; in that thejacket is welded such that a closed tube is formed; in that thelaminated wire structure formed of the wire cord and its jacket is drawnto a smaller diameter; and in that the drawn laminated wire structure isdivided into disks that form the individual resistors; wherein sections(14′) of the jacket that are located opposite one another on the diskcircumference are removed from each individual resistor together withthe respectively adjacent section (10″) of the resistive alloy such thatjacket sections 914) that serve as connector contact elements remainonly at the two ends of the remaining resistive element (10).
 9. Methodaccording to one of claims 4-8, characterized by the fact that the drawnlaminated wire structure is annealed in order to realize a diffusionlayer between the metal of the connector contact elements and thealloyed metal.
 10. Utilization of the resistor according to one of thepreceding claims for measuring currents.
 11. Utilization of a resistoraccording to one of the preceding claims as an SMD component.